1. Field of the Invention
The present invention relates to an inkjet recording head for ejecting ink to perform recording.
2. Description of the Related Art
Along with spread of a copying machine, a communication apparatus and an information processing apparatus such as a word processor and a personal computer, an inkjet recording apparatus for recording using an inkjet system has been developed as one of output devices for recording (printing) images for those apparatuses. An inkjet recording apparatus has advantages that an inkjet recording head (hereinafter, also simply referred to as a recording head) serving as recording means can be easily made compact, and that highly precise images can be recorded at a high speed. In addition, recording can be performed on plain paper without requiring special processing, and thus running costs are low. Moreover, noise during recording is low, since the inkjet recording apparatus employs a non-impact method. Furthermore, color-image recording is easily performed by using inks of several kinds of color tones (colors and/or concentration).
Recently, along with spread of the inkjet recording apparatus having these advantages, higher precision and higher speed of recording operations have been desired. To meet these demands, a recording head composed of a large number of densely arranged ejection openings is used in the inkjet recording apparatus. Moreover, in an inkjet recording apparatus capable of color recording, a recording head has a plurality of ejection opening arrays disposed corresponding to a plurality of color inks.
As the type of the inkjet recording apparatuses, there are a so-called line printer type and serial printer type. The latter is mainly used as a printer for personal or office use because of its relatively small size. In the serial printer type, main scanning and sub scanning are alternately performed to form an image. More precisely, in the main scanning, ink is ejected while the recording head is moved relative to a recording medium in a direction different from a direction of the ejection opening array. Meanwhile, in the sub scanning, the recording medium is moved relatively in a direction perpendicular to the main scanning direction. In the serial printer type inkjet recording apparatus, recording operations at a higher speed is achieved by performing bidirectional recording in which recording operations are performed in main scanning both in the forward and backward directions.
However, when a bidirectional color recording is performed using the recording head in which the ejection opening arrays for ejecting multiple colors of inks, for example, a cyan (C), a magenta (M) and a yellow (Y) are disposed in the main scanning direction, the order of ejecting these inks differs between the forward and backward directions of the main scanning. Accordingly, the order of applying these inks to the recording medium differs between the forward and backward directions of the main scanning. Consequently, a secondary color is not developed uniformly, and this causes unevenness in the secondary color having stripes with different color tones.
In order to deal with this problem, a technique is known in which ejection opening arrays for colors are disposed symmetrically in a recording head. For example, Japanese Patent Laid-open No. 2001-171119 discloses a structure in which an ejection opening array for C, an ejection opening array for M, an ejection opening array for Y, and another ejection opening array for Y, another ejection opening array for M and another ejection opening array for C are disposed in this order in the main scanning direction, and thereby the order of the color disposition is symmetrical. By use of the recording head with such disposition, the bidirectional color recording can be performed in the same order of applying the inks in the forward and backward directions of the main scanning. Thus, the secondary color can be developed uniformly.
On the other hand, ink droplets which are ejected from a recording head and adhere to a recording medium spread on the recording medium and form dots. Image is recorded as an assembly of the dots. The area per dot depends on the size of droplet, i.e., the amount of ejected ink. To achieve high image quality recording equivalent to a silver salt photography with high precision by use of the inkjet method, there is a trend that an ink droplet ejected from a recording head is made as fine as possible.
As a method for achieving such high precision recording, a technique is known in which an image is formed by combining dots formed of droplets with different sizes (different amounts of ejected ink). According to this method, it is possible to arrange dots with different diameters in an image, and thereby an image can be recorded by forming dots with relatively small diameters on a part of the image in which granular impression is likely to be noticeable, and by forming dots with relatively large diameters on a “solid” part of the image. Accordingly, the granular impression of the image is reduced, while the wide area of the “solid” part can be filled efficiently with a small number of ink ejections. Thus, high image quality recording can be performed at a high speed.
It is expected to achieve a high image quality recording at a higher speed by employing a symmetrical disposition of ejection opening arrays suitable for the aforementioned bidirectional recording in a recording head having a structure capable of ejecting different amounts of ink.
FIG. 16A shows a schematic plan view of the inkjet recording apparatus showing such a structural example. The recording head is formed on a Si substrate 10. On the substrate 10, five ink supplying ports denoted by the reference numerals 131 to 135 are disposed in the main scanning direction in parallel manner. Here, the ink supplying ports 131 and 135 correspond to a cyan ink. The ink supplying ports 132 and 134 located inner sides of ports 131 and 135 correspond to magenta ink. The ink supplying port 133 located in the center of the five ink supplying ports corresponds to yellow ink. To each of the ink supplying ports, ejection opening arrays and ink paths are provided. In the ejection opening arrays, a number of ejection openings are arranged in the sub scanning direction with a predetermined density (600 dpi (dots per inch)). The ink paths communicate with each ejection opening. In other words, the inkjet recording head is constructed symmetrically in the recording scanning direction in terms of the color order. The recording medium is to be provided with inks in the order of a cyan, magenta and yellow either in the forward scanning direction or in the backward scanning direction. In a part of the ink paths, energy generating element such as electrothermal transducer element (heater) is formed, and a driving signal is supplied via electrode portions 12 formed on the edge of the substrate.
On both sides of the ink supplying ports 131, 132, 134 and 135, ejection opening arrays CL1, ML1, ML2 and CL2 which eject relatively large amounts of ink, and ejection opening arrays CS1, MS1, MS2 and CS2, which eject relatively small amounts of ink, are disposed, respectively. On the other hand, on both sides of the ink supplying port 133, ejection opening arrays (YL1 and YL2) which eject relatively large amounts of ink are disposed. Here, as to the yellow ink, only the ejection opening arrays which eject relatively large amounts of ink are disposed. This is because the yellow ink has relatively low visibility as compared to the cyan ink and the magenta ink, the granular impression thereof is not substantially influenced even by the larger dots. Consequently, the effect of reducing the droplet size is small.
In the relation between the ejection opening arrays which eject relatively large amounts of ink in each color, the ejection openings are offset by ½ of the arrangement pitch in the sub scanning direction, and have a relation to complement one another, achieving a recording resolution of 1200 dpi. Moreover, as to the ejection opening arrays which eject relatively small amounts of the cyan ink and the magenta ink, the same relation is established.
In such a recording head, as to a cyan and a magenta, image with a recording density of 1200 dpi can be formed by use of large and small dots. Meanwhile, as to yellow, image with a recording density of 1200 dpi can be formed by use of large dots. Moreover, when recording is performed, especially emphasizing the speed to a plain paper sheet, the bidirectional recording can be performed on the same image area by use of only the ejection opening arrays which eject relatively large amounts of ink. At this time, since the ejection opening arrays for the same color ink are symmetrically disposed, the same order of applying inks in the forward and backward directions of the main scanning, and thereby it is possible to prevent the unevenness in the secondary color from occurring. Furthermore, for example, by performing the multiple main scanning (multi-pass recording) in accordance with the pixel arrangement complementary to the same image area, while effectively utilizing the ejection opening arrays which eject relatively small amounts of ink, it is possible to form a highly precise image with less granular impression.
However, when the present inventor has examined the above recording head, it is found that the symmetry disposition irrespective of the amount of the ejected ink causes the following problems. Hereinafter, descriptions will be given of the problems.
The recording head is positioned to a guide shaft of the recording apparatus via a number of members, i.e. carriage and other plural of components, and the main scanning is performed. Thus, as shown in FIG. 16A, if each ejection opening array is disposed accurately vertical to the guide shaft, the ejection opening arrays apart from each other (in this case, for the cyan ink, the ejection opening arrays CL1 and CL2, and the ejection opening arrays CS1 and CS2, for example) can complement each other. In reality, however, the recording head or the carriage may have a variation in the production, thereby the recording head is inclined to some extent, and the ejection opening arrays may not be completely perpendicular to the guide shaft.
FIG. 16B is an explanatory view of the state described above, showing the recording head which is inclined to the extending direction of the guide shaft, i.e. the main scanning direction by an angle θ. Due to such an inclination, the ejection openings in the ejection opening arrays CS1 and CS2, which should have a distance of approximately 21 μm ( 1/1200 inch) in the sub scanning direction, are further shifted by approximately 11 μm ( 1/2400 inch).
FIGS. 17A and 17B are schematic views showing dot formations corresponding to the ejection opening arrays for the cyan ink shown in FIGS. 16A and 16B, respectively. In FIGS. 17A and 17B, each of the drawings on the left shows the arrangements of dots c11 and c12 having relatively large diameters, which are formed by the ejection opening arrays CL1 and CL2 which eject relatively large amounts of ink, respectively. On the other hand, each of the drawings on the right shows the arrangements of dots cs1 and cs2 having relatively small diameters, which are formed by the ejection opening arrays CS1 and CS2 which eject relatively small amounts of ink, respectively.
In FIG. 16A, each ejection opening array is mounted completely perpendicular to the guide shaft. Thus, the ejection opening arrays CL1 and CL2 as well as CS1 and CS2, which are apart from each other, complement each other. As a result, the dots which are not shifted are formed as shown in FIG. 17A.
However, In FIG. 16B, the ejection openings in the ejection opening arrays located at a distance are shifted by more than the regular pitch. As a result, the shifted dots are formed as shown in FIG. 17B.
In this respect, if the ejection amount is sufficiently large, the formed dot diameter is also sufficiently large relative to the shifted distance as shown in FIG. 17B. Thus, the change of the area factor (the coverage of dots to a recording medium) is small in the sub scanning direction, and the influence thereof can be ignored. However, as to the ejection opening arrays which eject relatively small amounts of ink, the formed dots are small as shown in FIG. 17B. Thus, the ratio of change in the area factor to the sub scanning direction is relatively large.
The ratio of change in the area factor described herein is determined by the relation between the pitch in the ejection opening arrangement and the dot diameter. It becomes a problem when the dot diameter is small relative to the pitch in the ejection opening arrangement. Described above has been the case of arranging the ejection openings with a density of 1200 dpi. However, the same phenomenon would occur in a case of other arrangement density.
As described above, in the recording head shown in FIG. 16B, when the ejection opening arrays which eject relatively small amounts of ink are used for performing highly precise recording, large variations in an optical density may appear in the sub scanning direction, resulting in a problem that the stripes are more likely to be noticeable in the main scanning direction (horizontal direction). Moreover, the longer the shifting distance, the longer the distance between the ejection opening arrays in the main scanning direction. For this reason, the influence of the variations in the optical density is relatively increased in the order of yellow, magenta and cyan, resulting in a problem that the color balance may deteriorate as a whole.
Although the problems caused by the static shift has been described, dynamic factors such as vibration of the carriage or the guide shaft at the time of main scanning may cause the states shown in FIGS. 17A and 17B to occur repeatedly because of the difference between the positions of the aforementioned ejection opening arrays in the main scanning direction. In other words, when the ejection opening arrays which eject relatively small amounts of ink are used, the influence of variations in the optical density may increase due to the difference between the positions in the main scanning direction, resulting in a problem that strips may occur in the sub scanning direction (vertical direction).